Electricity transmission and distribution networks (or grids) must balance the generation of electricity with demand from consumers. At present, this is normally achieved by modulating a generation side (supply side) of the network by turning power stations on and off and/or running some power stations at reduced load. As most existing thermal and nuclear power stations are most efficient when run continuously at full load, balancing the supply side in this way results in an efficiency penalty. It is expected that significant intermittent renewable generation capacity, such as wind turbines and solar collectors, will soon be introduced to the networks, and this will further complicate the balancing of the grids by creating uncertainty in the availability of portions of the generation side.
Power storage devices and systems typically have three phases of operation: charge, store and discharge. Power storage devices typically generate power (discharge) on a highly intermittent basis when there is a shortage of generating capacity on the transmission and distribution networks. This can be signalled to the storage device operator by a high price for electricity in the local power market or by a request from the organisation responsible for the operating of the network for additional capacity. In some countries, such as the United Kingdom, the network operator enters into contracts for the supply of back-up reserves to the network with operators of power plants with rapid start capability. Such contracts can cover months or even years, but typically the time the power provider will be operating (generating power) is very short. In addition, a storage device can provide an additional service in providing additional loads at times of oversupply of power to the grid from intermittent renewable generators. Wind speeds are often high overnight when demand is low. The network operator must either arrange for additional demand on the network to utilise the excess supply, through low energy price signals or specific contracts with consumers, or constrain the supply of power from other stations or the wind farms. In some cases, especially in markets where wind generators are subsidised, the network operator will have to pay the wind farm operators to ‘turn off’ the wind farm. A storage device offers the network operator a useful additional load that can be used to balance the grid in times of excess supply.
For a storage system or device to be commercially viable the following factors are important: capital cost per MW (power capacity), capital cost per MWh (energy capacity), round trip cycle efficiency and lifetime with respect to the number of charge and discharge cycles that can be expected from the initial investment. For widespread utility scale applications it is also important that the storage device is geographically unconstrained i.e. it can be built anywhere, in particular next to a point of high demand or next to a source of intermittency or a bottleneck in the transmission and distribution network.
One such storage device technology is the storage of energy using a cryogen (Liquid Air Energy Storage (LAES)), such as liquid air or liquid nitrogen, which offers a number of advantages in the market place. Broadly speaking a LAES system would typically, in the charge phase, utilise low cost or surplus electricity, at periods of low demand or excess supply from intermittent renewable generators, to liquefy a working fluid such as air or nitrogen during a first liquefaction phase. This is then stored as a cryogenic fluid in a storage tank during a storage phase, and subsequently released to drive a turbine, producing electricity during a discharge, or power recovery, phase, at periods of high demand or insufficient supply from intermittent renewable generators.
LAES systems are predominantly mechanically based, with the main system components being turbo-expanders, compressors and pumps. Although these components can deliver response times of a few minutes, the response is not typically instantaneous.
LAES systems often include thermal storage to store the heat produced by the compressors used in the refrigeration cycle required to charge the system. This heat is then used to superheat the working fluid (i.e. cryogen) during the power recovery phase, increasing the amount of energy that may be recovered. Waste heat may also be stored from a co-located process.
During the storage phase, although the thermal storage is thermally insulated, heat egress occurs, causing a small portion of the thermal energy to be lost to the surrounding environment.
It would therefore be advantageous to improve the instantaneous response of a LAES system while also mitigating the effects of heat egress from the thermal storage and supplying further heat for increasing the output of the LAES system.